Program-controlled milling and drilling machines for mass-machining workpieces are often equipped with pallet changers that make possible a rapid exchange of the workpieces and thus increase the cutting performance while reducing down times. The pallets are equipped with workpieces outside the machine tool and releasably fixed on a pallet carrier of a pallet changer. The pallet is positioned above the workpiece table by way of a rotational movement of the pallet changer, and is clamped on this table after a lowering movement. A pallet of the pallet changer is fixed by way of a lifting movement and the resulting engagement of a hook-shaped or claw-shaped pallet carrier in a corresponding recess on the outer edge of the pallet.
FIG. 1 shows an example of the basic construction of a machine tool 1 of this type, such as is known, for example, from EP 2 047 946 B1, which comprises a machine bed 2, on which a pallet changing device 4 is arranged comprising a changing means 15 comprising claws 14 for gripping pallets in the course of the pallet changing process, and comprises a slide 5, which carries a workpiece table 6 comprising a first pallet 7 for clamping a workpiece for processing by a work spindle 9. A pallet carrier 12 is provided for carrying a changing pallet (not shown) comprising a second workpiece which is to be exchanged with the first pallet 7. Guide rails 3 are mounted on the machine bed 2, and the slide 5 is displaceable along the guide rails 3 in the X direction along the machine bed 2. The pallet change device 4 is arranged on an end of the guide rails 3 remote from the spindle 9, and fixed laterally on the machine bed 2 by way of a carrier arm 11. A motor 13 for rotating the changing means 15 through 180.degree. in an X-Y working plane for exchanging the pallets is mounted directly on a carrier plate of the changing means 15.
If the workpiece that is fixed on the changing pallet, instead of the workpiece which is fixed on the first pallet 7, is to be machined by the work spindle 9, the first slide 5 travels along the first guide rails 3 towards the pallet changing device 4 until a changing position is reached, in which the distance and position between the circular table 6 and the pallet carrier 12 are set in such a way that the changing means 15 of the pallet changing device 4 can engage both in the first pallet 7 and in the changing pallet. Subsequently, by raising the changing means 15, rotating the changing means 15 about an axis parallel to the Z direction, and lowering the changing means 15, the first pallet 7 is replaced with the changing pallet. The slide 5 is subsequently displaced along the guide rails 3 towards the spindle 9, together with the changing pallet located on the turntable 6, in such a way that the workpiece that is clamped on the changing pallet can be machined unimpeded.
FIGS. 2 and 3 illustrate in greater detail two stages of the above-described pallet changing process on a machine tool.
FIG. 2 shows a stage in which the claw elements 14 of a changing means 15 engage in receiving elements 16 both of the first pallet 7 and of a second pallet 8 (changing pallet). For this purpose, the circular table 6 and the pallet carrier 12 are located in the changing position, which is to say, at a predefined orientation and distance with respect to one another.
Subsequently, the first pallet 7 is swapped with the second pallet 8 by means of the changing means 15, that is to say, the second pallet 8 is now placed on the circular table 6 instead of the first pallet 7. This is carried out by raising and rotating the changing means 15 about an axis D, which is orientated parallel to the Z direction. Subsequently, the changing means 15 is lowered in the Z direction again, so as to release the operational connection between the claw elements 14 and the corresponding receiving elements 16 of the first pallet 7 and of the second pallet 8. Subsequently, the changing means 15 is again rotated about the axis of rotation D thereof into a home position, so as to reach the stage shown in FIG. 3. Subsequently, the slide 5 can be moved away from the pallet changing device 4 along the guide rails 3, so as to ensure unimpeded machining of the workpiece that is attached to the second pallet 8.
It is to the inventor's credit to have recognised that the dimensions of the changing means 15 in the X-Y plane are relatively large in the machine tool shown in FIGS. 2 and 3. This drawback is in particular due to the fact that an electric drive 13, which is necessary so as to rotate the changing means 15 about the axis of rotation D in the X-Y plane, is integrated into the changing means 15 itself. As a result, in the stage of the pallet changing process shown in FIG. 3 (that is to say, in the changing position), it is only possible to a limited extent to move the circular table 6 as desired with respect to the changing means 15, in particular, to pivot the plane of the workpiece table, this being a basic requirement when a pivoting circular table is used for 5-axis machining. Disadvantageously, before the circular table 6 can be rotated or pivoted, the slide 5 first has to be displaced along the guide rails 3 of the pallet changing device 4 towards the work spindle, together with the workpiece table. However, this takes time and reduces the available workspace of the machine tool.